One of the main factors to quantify the quality of a printed image is the tone scale representation, expressed by the optical density range and the exactness and stability of the contone rendering. In a digital printing machine, such as an electrophotographic engine, each tone of a contone image is produced by a certain spatial combination of some or all of the available tones per pixel. This process is referred to as screening. The set of tones, available in the machine, is defined by the properties of the exposure device. For instance, in an electrophotographic printer that uses a binary exposure device, only two tones (black and white) are available to the screening algorithm to reproduce a contone image. In some machines however, multiple tone levels are available to the screening process by applying area or intensity modulation on the output spot of the exposure device (see below). As screening is well-defined and, by its nature, perfectly repeatable, the image quality of the engine is largely determined by the ability to reproduce the set of tones. In an electrophotographic engine the contone density of each microdot is determined by the mass of toner per unit area transferred to paper. This toner mass, referred to as M/A and expressed in mg/cm.sup.2, is a function of an almost limitless amount of parameters. Most of these parameters can be regarded as fixed by design and thus invariable during the operation of the engine. Some however are extremely variable. The most important in a two-component developer system are:
toner concentration (TC)=the ratio of the amount of toner and the amount of carrier available in the developing unit in a two-component system. PA1 toner charge per unit of mass (Q/M), expressed in .mu.C/g. PA1 development potential (V.sub.DEV), expressed in Volt=the potential difference V.sub.E -V.sub.B over the development gap between the developer supply roller (bias voltage V.sub.B) and the photosensitive element (voltage after exposure V.sub.E) upon which a latent image is present. The photosensitive element is mostly implemented as an Organic Photoconductor or OPC. PA1 transfer efficiency (TE), expressed in %: the ratio of the amount of toner transferred to the printing medium and the amount of toner developed on the photosensitive element. This dependency can be formally expressed as: EQU M/A=f(TC, Q/M, V.sub.DEV TE) PA1 the triboelectric properties of toner and carrier, PA1 toner concentration TC, PA1 relative humidity RH of the air in the developing unit, PA1 agitation of developer in the developing unit. PA1 the initial charge level V.sub.C of the OPC, PA1 the bias voltage V.sub.B applied to the toner supply roller of the developing unit and PA1 the intensity E.sub.EXP of the image dependent illumination of the photosensitive element. PA1 toner charge Q/M, PA1 amount of toner on the photosensitive element and PA1 the value of the electric field in the transfer zone. PA1 extremely low toner charge Q/M at high relative humidity RH, leading to an increase in dust production, fogging and possibly inconsistent transfer quality over the whole tone scale. PA1 extremely high toner charge at low relative humidity, decreasing the developability of the toner. This requires large electric fields in the developing stage and consequently implies more powerful engine hardware.
and is generally referred to as the developability and transferability f() of the toner.
In an electrophotographic engine, the reproduction of multiple tones is highly sensitive to each of these variables. Toner concentration TC changes during engine operation due to depletion of toner caused by image development and toner addition under control of the engine. Toner charge Q/M is determined by:
When the developer is properly agitated, an unambiguous relationship can be found between Q/M, TC and RH. The development potential V.sub.DEV is determined by:
Transfer efficiency TE on its turn is, amongst other factors, determined by:
Present electrophotographic machines maintain the optical density of their produced tones by keeping toner concentration TC at a constant level. For this purpose they use a toner concentration sensor in the developing unit, or a density sensor that measures the density D.sub.OPC developed on the OPC, or both. Changes of the toner charge Q/M, due to relative humidity RH or variations of RH are compensated for by changing the development potential V.sub.DEV and the value of the transfer electric field. Disadvantages of this technique are:
Furthermore, it can be shown that for a two-component developing system, the development of the latent image is almost purely driven by toner charge Q/M. Therefore toner charge Q/M would be a valuable input to any process control system for steering the electrophotographic process. Generally, online toner charge measurement Q/M can not be implemented easily without the need for high precision measurement hardware, which leads to an increase in system variable cost. As stated before, producing several tones in an electrophotographic engine can be done by area modulation or by intensity modulation of the light beam of the exposure device (or by any combination of both). In this way, a set of microscopic tones at the pixel or microdot level are created. These form a microscopic gradation that has to be kept constant for the contone rendering, handled by the screening process, to be repeatable.